Maximum circuit length calculator
Longest run that voltage drop and the fault loop allow.
Reads AS/NZS 3008.1.2:2017 (NZ) and AS/NZS 3000:2018. Tables checked 26 Sept 2026.
Try a job 6
Each job fills in example values. Replace them with your values.
- Find the longest run for a lighting circuitThe longest route a 1.5 mm² lighting circuit on a 10 A breaker can take.
- Find the longest socket circuitThe longest run for a 20 A circuit on 2.5 mm².
- Find the longest oven circuitThe longest run for a 32 A circuit on 6 mm², by voltage drop.
- Find the longest motor circuitThe longest three‑phase run on a 16 A D‑curve breaker.
- Find the longest run at 3 %The longest run when the drop limit is 3 %.
- Find the longest heat pump runThe longest run for a 16 A C‑curve circuit on 2.5 mm².
Inputs
A worked example with the default inputs.
- Active conductor
- 2.5 mm²
- Earth conductor
- 2.5 mm²
- Insulation
- PVC
- Protective device
- Type B circuit-breaker
- Device rating
- 20 A
- Phases
- Single phase
- Nominal phase voltage
- 230 V
- Design current
- 20 A
- Power factor
- 0.9
- Voltage drop limit
- 5 %
Maximum route length
35.26m
- Fault loop route length
- 128m
- Voltage drop route length
- 35.26m
- Governing check
- Voltage drop
This result is a design aid. Verify it against the current standard.
Calculation steps
- Fault loop route length128m
AS/NZS 3000:2018 Table B1
- Resistance of the copper conductor at operating temperature9.01Ω/km
AS/NZS 3008.1.2:2017 Table 30, Tables 34 to 35
- Reactance0.1Ω/km
AS/NZS 3008.1.2:2017 Table 30, Tables 34 to 35
- Voltage drop per amp metre16.31mV/A·m
AS/NZS 3008.1.2:2017 Table 30, Tables 34 to 35
- Voltage the drop limit is taken from230V
AS/NZS 3000:2018 Cl 3.6.2
- Voltage drop limit11.5V
AS/NZS 3000:2018 Cl 3.6.2
- Voltage drop route length35.26m
AS/NZS 3000:2018 Cl 3.6.2
- Governing lengthResult35.26m
AS/NZS 3000:2018 Cl 3.6.2
About this calculator
How this is calculated
The tool finds the longest final subcircuit route that two limits each allow. It reports which limit governs. It reads AS/NZS 3000:2018 and AS/NZS 3008.1.2:2017.
- Fault loop length. The tool reads the route length for your breaker, size and earth size from Table B1.
- Resistance and reactance. It reads these for the active conductor. The tables are Table 30 and Tables 34 and 35, for copper at operating temperature.
- Millivolts per ampere metre. It combines both values with your power factor.
- Voltage drop limit. Cl 3.6.2 sets the limit. You enter it as a percentage of the supply voltage.
- Voltage drop length. It finds the route length at which the drop equals the limit for your design current.
- Governing length. The shorter of the two lengths is the maximum route length.
| Step | Table or clause | What it decides |
|---|---|---|
| Fault loop length | AS/NZS 3000:2018 Table B1 | The longest route for the breaker to trip |
| Cable data | AS/NZS 3008.1.2:2017 Tables 30, 34 and 35 | Resistance and reactance |
| Voltage drop length | AS/NZS 3000:2018 Cl 3.6.2 | The longest route within the drop limit |
Worked example
- 2.5 mm² on a 20 A type B breaker. The earth is 2.5 mm², with PVC insulation, on a single‑phase 230 V supply. The design current is 20 A at a power factor of 0.9. The fault loop route length is 128 m. The voltage drop route length is 35.26 m. The voltage drop governs, so the maximum route length is 35.26 m.
Limits
The tool assumes copper conductors. The reactance is the Table 30 value for a multicore cable. A single‑core run reads a higher reactance for its formation.
The tool does not check the current rating of the cable. It does not add correction factors. Use the cable size calculator for the full check.
See also: Maximum length of a 20 A final subcircuit, The voltage drop limit in Cl 3.6.2 and the voltage drop calculator.
Questions
The result is a design aid. Verify against the current standard.
What does the maximum circuit length tool do?
The tool reports two lengths. The first is the route length that still allows the protective device to disconnect in time. The second is the length at which the voltage drop reaches your limit. The shorter of the two governs the run.
Where do the two lengths come from?
The fault loop length comes from AS/NZS 3000:2018 Table B1 for a 0.4 s disconnection time at 230 V, scaled by your nominal voltage. The voltage drop length uses the resistance and reactance from AS/NZS 3008.1.2:2017 Tables 30, 34 and 35, against Clause 3.6.2.
Why does a combination show no fault loop length?
Table B1 tabulates a fixed list of conductor pairs and device ratings. A pair that is not printed has no tabulated length, so the tool reports the voltage drop length only. Use the loop impedance tool to check that combination instead.
Clauses and tables this tool reads
- B1Maximum route lengths for conductor sizes and protective devices
- AS/NZS 3008.1.2:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
Every clause and table, with how each was verified
Tables last checked on 26 Sept 2026.
Related tools
- Cable size calculatorFind the smallest cable size that passes each check.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Voltage drop calculatorCalculate voltage drop on a cable run and check the limit.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Earth fault loop impedance calculatorCheck earth fault loop impedance against the maximum Zs.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
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